The widespread use of optical fibers (lightguides) in telecommunications, medicine, and metrology can, in general, be directly related to the advent of low-loss glass fibers. Though some loss is inherent, low loss fibers result from reducing the loss mechanisms incorporated in the fiber during manufacture. These mechanisms include, among others, impurities that act as light absorbers, and geometrical distortions that lead to scattering of light outside the fiber. Additionally, widespread deployment of low loss optical fibers has generally required that fibers possess the material strength to withstand placement in harsh environments, which has been facilitated by reducing structural faults, such as bubbles or chemical impurities, that can cause significant mechanical stress and weakening of the fiber and/or can cause added loss. Typically, the loss mechanisms and structural faults in an optical fiber drawn from a glass preform result from these imperfections existing in the glass preform. Thus, to manufacture high strength, low loss glass fibers efficiently, techniques must be employed that reduce the loss mechanisms and structural faults present in the glass preform.
When loss mechanisms and structural faults result from preform surface imperfections, they can be substantially eliminated by removing surface material comprising the imperfection (this removal process being referred to as an etching process). Etching techniques, such as mechanical milling and chemical etching, are available that can be applied to glass preforms. Conventional chemical etching is relatively slow and is typically not a clean process. Though some imperfections are removed by chemical reaction, different imperfections can be incorporated as a byproduct of the etching reaction. In addition, chemical etching is typically isotropic, which is generally not suitable for selectively removing preform surface material. Mechanical milling is adaptable to the normal processing environment, but can introduce mechanical stress into the glass preform, and can lead to preform structural failure, e.g., formation of cracks that can propagate through the perform.
Additionally, the shortcomings associated with chemical etching and mechanical milling to remove surface imperfections also make these etching techniques undesirable for eliminating eccentricity in glass preforms. Eccentricity of a glass preform occurs when one or more of the three cross-sectional regions (core, deposited cladding, and substrate), of which the preform is typically comprised, deviate from a desired concentric configuration, resulting typically in geometrical distortions in the optical fiber. Deviations from a desired optical fiber design can generally lead to deviations from the fiber's desired light transmission characteristics. For example, light transmission through a passively aligned connector between two fibers can result in substantial loss if one of the fibers is eccentric, since the connector typically aligns the cross-sectional perimeters of the fiber ends which, due to eccentricity of at least one of the fibers, results in misaligning the fiber cores. Core alignment is generally crucial for providing low loss fiber connections. Therefore, a method of eliminating, or reducing to acceptable levels, preform eccentricity would be highly advantageous for the production of optical fibers that can be deployed in a conventional manner with little or no loss due to eccentricity.